Method of producing polychromatic light



Nov. 1, 1938.

METHOD OF PRODUCING POLYCHRONLATIC LIGHT Filed Oct. 1, 1956 InventorCliFton 6. Found,

His A torney.

c. G. FOUND 2,135,283

Patented Nov. 1, 1938 UNITED STATES PATENT OFFICE Clifton G. Found,Schenectady. N. Y., assignor to General Electric Company, a corporationof New York Application October 1, 1936, Serial No. 103,587

2Claims.

The present invention relates to a novel method of operating luminousdischarge devices of the type known as cathodic lamps.

It is an object of the invention to provide an i improved manner andmeans of operating cathodic lamps containing a mixture of' gases orvapors whereby the characteristic spectra of each of said gases orvapors shall be sequentially emitted in such a fashion as to produceeither blended or continuously variable lighting.

Briefly stated, this object is attained by so regulating the cathodetemperature at a given discharge current, or by so regulating thedischarge current at a given cathode temperature, that at least duringcertain portions of the operating cycle of the lamp different componentsof the gaseous mixture in the lamp are caused to be preferentiallyexcited to give light, thus varyin the character of the instantaneouslight output 0 of the lamp. Since different gases emit different coloredradiations when excited, the effect of these variations will be toproduce the appear- .ance either of blended polychromatic light or of alight of constantly varying shade, depending 5 on the rapidity withwhich the variations are caused to occur.

The novel features which I desire to protect herein will be pointed outwith particularity in the appended claims. The invention itself, how- 0ever, will best be understood by reference to the followingspecification taken in connection with the drawing, in which Fig. '1illustrates schematically a lamp and energizing circuit thereforsuitable for the practice of the invention, and Figs.

,5 2 and 3 are graphical representations useful in explaining theinvention.

It is generally known that the luminescence produced by a gaseousdischarge between relatively spaced electrodes is comprised of a plu- L0rality of zones having somewhat different characteristics. In such adischarge the principal sources of light comprise the positive columnwhich forms in the intermediate space between the electrodes and thecathodic glow which L5 occurs in a region very close to the cathode. Itis also known that conditions of operation may be chosen such thatessentially no positive column is present while the cathodic glow israised to a maximum. In general, if the spacing between 50 the cathodeand anode is equal to or less than the least dimension of the dischargevessel within. which the electrodes are enclosed, the light producedwill be predominantly of a cathodic glow nature. For this reason lampshaving a 55 construction such as that specified are commonly referred toas cathodic lampsand will be so referred to herein.

In order to understand clearly the nature of my invention it isnecessary to refer briefly to the principles governing the generation oflight 5 in a cathodic lamp. *In suchlamps the applied potential isconcentrated as a cathode drop in a narrow sheath, perhaps 6 centimeterin thickness, directly surrounding the cathode and it is in this sheaththat electrons emanating from the 10 cathode are accelerated. Theacceleration actually produced depends on the conditions of operationand for dischargecurrents which are low with respect to the emission ofthe cathode will be on the order of the ionization potential of the gasor vapor which is carrying the discharge.

Electrons proceeding from the boundary of the sheath toward the anodewill produce such ionization of the'gaseous atmosphere as is required tomaintain a plasma or condition of equilibrium in the discharge space. Asis well known, ionization consists in the creation of positively chargedparticles and is a result of collisions between atoms and electrons andthe absorption of energy by the former. Such collisions as do not resultin the production of positive ions may'produce excitation of thecolliding atoms, the electron energy required for excitation being lessthan that required for ionization. It is this latter phenomenon(excitation) which is chiefly responsible for the production of light.

It may be shown that in a gaseous discharge device the rate ofproduction of positive ions is determined jointly by the electroniccurrent moving from the cathode and by the average velocity of theelectrons which comprise that current. In a cathode discharge device theratio Ie/Ip, in which 10 represents the electronic .current and 19represents positive ion current moving to the cathode, will remain aconstant maximum as long as the electrons demanded by the discharge arenot in excess of those supplied by the zero field emission of thecathode (i. e. the emission of which the cathode is capable without thepresence of a positive field at the cathode 5 surface). Otherwisestated, as long as the supply of electrons which will be generatedthermally by the cathode without the .assistance of an electrostaticfield at the cathode surface is equal to or greater than the electroniccurrent required by the discharge, each electron leaving the cathodewill produce a definite and constant number of ions which number isdetermined by the average electron energy (1. e. by the cathode drop).Consequently, during'the time that this condition is maintained anincrease in the discharge current will simply involve more of thethermally emitted electrons without requiring any change in electronvelocity or in the mag- ,nitude of the cathode drop. This will be true,

at least as long as the atoms of the gaseous constituent which iscarrying the discharge are present in sufiicient quantities to supplysubstantially all of the ions required by the discharge. Since this isthe usual condition, it is the one which will be assumed to exist in thefollowing discussion.

If, on the other hand, the magnitude of the discharge is such as torequire a greater number of electrons than are supplied by the zerofield emission oi the cathode, then a. compensatory change in conditionmust take place. This change is accomplished by an increase in electronemission from the cathode, which increase is produced by the action ofan electric field as will be more fully explained in the following:

The electric field required to offset the inadequacy of the electronsupply may be created either by an increased cathode drop or by theaccumulation of positive ions in the vicinity of the cathode. As amatter of fact both these factors wil be involved in theequilibrium-seeking changes necessitated by an increase in dischargecurrent with respect to the zero field electron emission of the cathode.This is true because an increased cathode drop, acting through anincrease in electron velocity, raises the rate of production of ions perelectron which in turn produces a greater electric field at the cathode.On account of the duofold nature of this adjusting process the cathodedrop will seek a value at which the rate of generation of positive ionsis exactly that which will produce the field regiuired to stimulatetherequired electron emis- It will be seen that} as a consequence of theincreased rate or production of positive ions with reference to electronemission the ratio Ie/Ip between these two quantities will havedecreased. As still greater fields are required for further increasingdischarge currents this ratio must again be decreased byadisproportionate change in the value of Ip. Thus, it will be seen thatwhereas throughout the range of zero field emission, a linear relationexists between the electron and positive ion currents, in the region ofrequired field emission, we have a non-linear relation between thesesame quantities.

It the discrepancy between the zero field emission of the cathode andthe electron current involved in the discharge is sufliclently great, aconsiderable increase in the cathode voltage drop will be noted, and thevelocity" imparted to the electrons passing through the cathode sheathwill be substantially enhanced. Assuming that equilibrium is to becontinually maintained, this velocity will increase with increasing arccurrent until a cathodedrop is reached at which ions are produced notonly from the gaseous constitu ents having lowest ionizing potentialsbut also from those having higher ionization potentials. Simultaneously,luminous excitation of these latter constituents sets in. This conditionbeing attained, the discharge will then tend to emit light whichcomprises a mixture in varying proportions oi the spectra oil-theseveral gases contained in the discharge space. i

The further significance of the principles described above and themanner of their practical application in accordance with the inventionmay be most readily understood by reference to the specific structureillustrated in the drawing.

Referring to Fig. l, I have shown a sealed envelope I, suitably ofglass, having at the lower end a reentrant stem 2 terminating in a press3. A transverse septum 4, for example of mica, serves substantially toisolate the stem-containing portion of the envelope'from the maindischarge space. Within the envelope I provide an ionizable gas, forexample, neon, at a pressure of from about one-half to severalmillimeters, for example, 1 millimeter, and a quantity of vaporizableeasily ionizable material indicated at 6 as adhering to the walls of thedischarge vessel, which latter material may suitably comprise a mixtureof sodium and mercury. It will be understood, of course, that othergases, for example, argon, and other vaporizing materials, for example,cadmium, zinc, or various amalgams may be alternatively employed; Ingeneral, the conditions of operation should preferably be such that theconcentrations of the various constituents are in inverse order to theirionization potentials. For example, in the case of sodium, mercury andneon, the conditions may suitably be such that the pressure of sodium isabout 0.1 micron, the pressure of-mercury about 0.1 millimeter and thepressure of neon about 2 millimeters.

An anode comprising a pair of similar sleevelike metal rings 8 and 9 issupported on a vertical rod secured at its lower end to the press 3.This rod may consist of a. conducting element l0 embedded for thegreater portion of its length in a refractory insulating material H,such as alumina, which serves to protect the conductor from the effectsof the discharge. The anode parts 8, 9, are electrically connected byconductor l0 and are consequently maintained at approximately the samepotential. Intermediate between 1 these equi-potential parts issupported a thermionic cathode l3 suitably comprising a refractory basemember, for example, of tungsten, coated with an electron emisslvematerial, for

example, barium oxide or thorla. The cathode is so positioned that itsdistance from the anode surface is less than the least dimension oftheenvelope 1', thus fulfilling the requisite conditions for themaintenance of a cathodic type 01' discharge. Heating current issupplied to the oathode through insulated lead-in connections I 4 and I;which also serve to support the cathode in place.

Externally of the envelope the lead-in connections I4 and I! which aresealed through the press 3, may be connected to a source of heatingcurrent. In the case illustrated this comprises atransformerhavingasecondary I 6 and a primary fl-gthe latter beingprovided with aserially connected current-11ml ng device 18 shown as a variableresistor? Ac. the anode and cathode terminals is im" a dischargepotential derived as shown from the secondary of a transformer 20. Hereagain, a. current-limiting device such as a variable reactor 22 isprovided in series with the transformer primary.

In the operation of a lamp such as that described, if the temperature ofthe cathode I3 is maintained sufliciently high to produce an adequatesupply of electrons, the ratio I /I. will be a minimum (or Ie/Ip amaximum, as previously explained) and the positive ion current will bepreponderantly produced by ionization of the sodium. This requireselectrons or a velocity equal to or slightly in excess or 5.1 volts andwill'res'ult in the production of light developed almost exclusively bythe excitation of sodium atoms.

Under the conditions stipulated substantially none of the electrons willpossess a velocity sumcient to excite either the mercury or the neon toformer, this condition may be considerably altered. When the cathodeemission is so reduced that during a portion of the discharge currentcycle the zero field supply-of electrons becomes insufficient tomaintain the discharge equilibrium, an increase in cathode drop willtake place in accordance with the principles explained above. As soon asthe cathode drop becomes equal to the excitation potential of mercury(6.7 volts) a portion of the light emitted by the discharge will havethe characteristics of the mercury spectrum. With a still furtheraccentuated discrepancy of electron supply and electron demand thecathode drop may increase to the point where even the neon becomesexcited.

Referring now to Fig. 2, I have shown graphically the nature of theresults which may be obtained when using an alternating currentdischarge supply in connection with cathodic lamps, employing mixedgases. In this figure the sinusoidal curve A represents the cyclicalvariations of the discharge current, and the horizontal dotted linesindicate the currents at which, for

cathode temperatures T and T2 respectively, the cathode drop exceeds theabove-mentioned excitation voltage of mercury. With the temperature T1,for example, the zero field emission of the cathode may be suflicient totake care of the demands of the discharge current in the time intervalab. However, as the current continues to increase, this condition nolonger obtains and an increase in the cathode drop sets in so that inthe interval be the light emission from the discharge may be partially,or even predominantly composed of mercury luminescence. Furtherrepeatedvery many times during each second,

according to the frequency of the current supply, the apparent effect onthe observer will be that of the generation of a mixed or blended lightcomprising the spectra of all three of the gaseous constituents. Whenthese constituents comprise sodium, mercury and neon, respectively,characterized by yellow, blue, and red luminescence, thecolordistribution may be such as to produce a polychromatic or approximatelywhite light.

Referring to the right-hand portion of Fig. 2, it will be seen that bychanging the cathode temperature to a higher value T2, the relativeproportions of each cycle during which the various gaseous constituentsare respectively most active may be varied at will. For example, in thecase tage of these facts it will be seen that one may vary at will thespectral distribution of the emitted polychromatic light by changing thecathode temperature to'produce the desired result.

Referring now to Fig. 3 the curves illustrated show how color regulationmay be obtained in a slightly difierent manner by adjustment of theaverage value of the discharge current while leav ing the cathodetemperature constant. In curve B, which represents the greater value ofthe dis- .charge current, the cathode is assumed to be set at atemperature T3 at which zero field emission is less than that requiredto supply the current at the peak of the cycle. Consequently, colorblending of the type previously described may occur. If, however, withthe same cathode tem-.

perature, the discharge current is reduced to the value indicated by thecurve C, no increase in cathode drop will occur at any time during thecycle. For this reason only sodium light will be apparent to theobserver.

' The method of color regulation described in the last paragraph alsolends itself readily to the production of shifting or mobile coloreffects.

By maintaining a discharge current (either alternating or direct) ofrelatively constant average value and cyclically varying the cathodeheating current in slow degrees, the color of the emitted light may bemade to change in a very pleasing manner. As the cathode temperaturefalls below that at which the zero field emission is less than theelectronic current required by the discharge, the spectrum of theemitted luminescence will be more and more that of the gaseousconstituent of highest excitation potential. On the other hand, as thecathode temperature again increases the spectrum of the constituents oflower excitation potential will again predominate. It will be clear thatthis mode of regulation may be accomplished either manually orautomatically by the use of a rotating contactor device or other knownmeans included in the cathode circuit, whereby the cathode current maybe cyclically varied in a desired position.

While the method which comprises my invention should be clear from thedescription set 'forth in the foregoingspecificationHI may point outthat its underlying principle is that the conditions of the dischargeshall be such that the required positive ion current is caused to varynon-linearly with the electronic current. If this condition isfulfilled, no matter what the cause of such-fulfillment, a change indischarge current will inherently produce a change in electronvelocities. In a cathodic lamp this result may be obtained by theprocedure which I have outlined in the foregoing. In other types ofdischarge lamps where the same principles of equilibrium do notnecessarily apply other conditions may be required to produce anonlinearity of the character referred to.

In the interests of simplicity I have explained my invention inconnection with a type of discharge in which single impact ionization isas sumed to be exclusively involved. However, in cases where ionizationis cumulative (i. e. produced by successive electron impacts) similarreasoning may be applied.

While I have described my invention in connection with a particularstructure, it will be understood by those skilled in the art-that manyother structures including lamps adapted to be conductive in bothdirections may be alternatively employed, and I aim by the appendedclaims.

to cover all such uses of my improved mode of operation as fall withinthe true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A method of producing color variations in a cathodic lamp containinga plurality of ionizing media which comprises passing a dischargecurrent of relatively constant average value through said lamp andcyclically varying the normal zero field emission of the cathode of thelamp in such a manner that such emission is less than the electroncurrent required by the discharge during a substantial portion of thevariation and is greater than said current during another substantialportion of such variation whereby sequential excitation of the variousionizable media is obtained.

2. A method of producing color variations in a cathodic lamp containinga plurality of ionizable media of difierent excitation and ionizationpotentials, which comprises passing a discharge current of relativelyconstant average value through said lamp and cyclically varying thetemperature of the cathode of the lamp in such a manner that the zerofield emission of the cathode is less than the electron current requiredby the discharge during a substantial portion of such variation and isgreater than such current during another substantial portion of thevariation whereby sequential excitation of the various ionizable mediais obtained.

CLIFTON G. FOUND.

